This project seeks to develop and evaluate agents targeted against T ell activation antigens as a possible method of preventing graft-versus-host disease (GVHD) after allogeneic marrow transplantation. When donor cells are activated, they express certain molecules ("activation antigens") that are not present on resting T cells. Among T cell activation antigens, the IL-2 receptor (IL-2R) represents an especially attractive target for immunosuppressive therapy, First, the biology of this receptor has been extensively studied, leading to a detailed understanding of its structure and function. Second, the IL-2R serves not only as a marker of T cell activation but also plays an essential functional role in the immune response. Thus there is little evidence to suggest that an immune response such as GVHD can be initiated by cells that do not express IL-2R. Third, with certain exceptions, cells other than activated by T lymphocytes do not express IL-@R. Thus cytotoxic agents specifically targeted against the IL-2R should be well tolerated. Finally, a variety of agents specific for human and murine IL-2R have become available for testing. Previous studies in mice and humans have shown that GVHD can be ameliorated by administration of antibodies specific for CD25 which represents the p55 component of IL-2R. Success has been limited because these antibodies do not kill all cells to which they bind. Thus this project will focus on ligand toxins and immunotoxins that can kill activated T cells, testing both in vitro and in vivo approaches. The specific aims are as follows: 1. IL-2 ligand toxins and CD25-specific immunotoxins will be assessed for their ability to kill "donor" murine T cells activated by "host" alloantigens in mixed lymphocyte culture (MLC). Efficacy will be tested by assays that measure viability, protein synthesis and proliferation of activated cells and the generation of cytotoxic T cells. 2> T cells from MLC treated with selected agents will be adoptively transferred into irradiated allogeneic recipients to determine whether treatment can ablate donor T cells reactive against host alloantigens in vivo. Assays will measure CD25 expression and blastogenesis of donor cells in the lymph nodes and spleen of recipients on days 2-6 after adoptive transfer. 3. As a additional approach, selected agents will be administered in vivo early after transplantation when IL-2R is first expressed. Subsequent proliferation of donor and host T cells will be monitored by measuring CD25 expression, and treatment will be repeated if necessary. 4. In vitro or in vivo treatments that ablate host-specific alloreactive proliferation of donor T cells in vivo will be further tested for their ability to prevent GVHD when T cells are adoptively transferred with donor marrow into irradiated allogeneic recipients. Selectivity for killing only the subset of donor R+T cells reactive against host alloantigens will be tested by determining whether immunity against an irrelevant antigen can be adoptively transferred from immunized donors into unimmunized allogeneic recipients. 5. Approaches that prevent GVHD will be evaluated to determine whether treatment interferes with engraftment. These experiments could provide the rationale and impetus for clinical testing of IL-2 ligand toxins or CD25-specific immunotoxins in human marrow transplantation.